Avoid compiler warnings about 'const char*'.

Modules/_ctypes/cfield.c

@@ -1637,7 +1637,7 @@ static struct fielddesc formattable[] = {
 */
 
 struct fielddesc *
-getentry(char *fmt)
+getentry(const char *fmt)
 {
 	static int initialized = 0;
 	struct fielddesc *table = formattable;

Modules/_ctypes/ctypes.h

@@ -128,7 +128,7 @@ extern PyTypeObject SimpleType_Type;
 #define SimpleTypeObject_Check(v)	PyObject_TypeCheck(v, &SimpleType_Type)
 
 extern PyTypeObject CField_Type;
-extern struct fielddesc *getentry(char *fmt);
+extern struct fielddesc *getentry(const char *fmt);
 
 
 extern PyObject *